1. Field of the Invention
This invention relates to gas turbine engines and particularly to a new and unique compound propulsor which efficiently incorporates within it a core gas turbine engine powering a fan and at least one propeller.
2. Description of the Prior Art
Several types of gas turbine engines are currently used to power aircraft. Two types of such engines are the turbofan and the turboprop. The turbofan engine employs a core gas turbine engine to power a fan, while the turboprop engine employs a core gas turbine engine to power a propeller.
A highly efficient type of turbofan engine is the high bypass ratio turbofan which employs a large diameter fan. The maximum performance benefits obtainable from a high bypass ratio turbofan occur during a different phase of flight than do the performance benefits from a turboprop. The performance and efficiency of a turboprop engine are superior at lower flight speeds, such as cruise in the Mach 0.5 to 0.65 range. At speeds above M=0.7, the performance of the turboprop deteriorates due to reduced propeller efficiency and high installation losses, that is, drag losses caused by the increased velocity of the air in the propwash flowing over or against aircraft surfaces. At such flight speeds, the relative helical velocity of the propeller tip becomes supersonic and the compressibility effects then reduce propeller efficiency and create undesirable undesirable aerodynamic noise. At high subsonic speeds, such as M=0.7 to 0.8 as would be encountered during the cruise phase of a typical modern commercial airplane flight, the performance and installed propulsive efficiency of a turbofan engine can exceed that of a turboprop which uses conventional propeller blades.
The thrust lapse rate, that is, the decrease in engine thrust with increasing altitude and airspeed, is a means of comparing engine performance.
The thrust lapse rate of a turboprop engine is greater than the thrust lapse rate of a high bypass turbofan engine. Because of such a high lapse rate, the propeller of a turboprop would be greatly oversized at low altitude/low speed conditions (e.g. takeoff) in order to provide the amount of thrust at high altitude/high speed conditions which would be avilable from a high bypass turbofan engine of equivalent core size. The extra weight and other disadvantages of an oversized propeller make the use of a turboprop at high altitude and high speed undesirable.
Likeise, at low air speeds, the fuel efficiency of an installed turboprop exceeds that of a turbofan. At high subsonic speeds, on the other hand, a high bypass ratio turbofan can be more fuel efficient than a turboprop, due to the above-mentioned propeller inefficiencies and reduced installation losses.
Many current aircraft designed for relatively short flight distances, such as between 200 and 1,000 nautical miles, selectively employ either turbofan or turboprop engines. When takeoff and landing performance is particularly important, due to short runway lengths or obstructions near an airport, for example, and noise suppression requirements can be met, turboprop engines are selected. When improved block time performance and fuel efficiency at high speed cruise is important, for example minimize flight time and fuel costs for passengers or cargo, high bypass ratio turbofan engines are selected. Yet the selection of one type of engine correspondingly results in some loss of the total performance benefits which would have been obtained from the type engine not selected.
A problem encountered with turboprop engines is propeller-generated noise. A turboprop engine tends to be noisier than a turbofan engine with an equivalent core size. The fan cowl which encases the fan and the fan duct in a turbofan engine can be acoustically treated to muffle noise, whereas the propeller of a turboprop engine extends into the airstream where noise muffling is difficult. Additionally, the level of noise generation of a turboprop increases with an increase in the rotational speed of the propeller and with an increase in propeller blade pitch. Such noise can be disturbing not only to the community over which the aircraft flies but also within the interior of the aircraft itself.
In view of the above-mentioned problems, it is therefore an object of the present invention to provide a compound propulsor which includes the efficiency and performance benefits of a turboprop engine yet which can develop adequate thrust at high altitude and airspeeds without the need for an oversized propeller.
Another object of the present invention is to provide a compound propulsor in which fuel efficiency is optimized throughout all phases of flight.
Yet another object of the present invention is to provide a compound propulsor which includes the performance benefits of a turboprop engine but in which the propeller generated noise can be selectively reduced.